1. Field of the Invention
This invention relates to apparatus and methods for spatially characterizing a particle beam, and more particularly to apparatus and methods for sensing the direction, location, convergence, aberrations and intensity profile of a neutral particle beam, and for controlling the beam propagation in response to one or more of the sensed characteristics.
2. Description of the Prior Art
Neutral particle beams may be generated with diameters ranging from 1 mm. to 100 cm. or greater, and correspondingly large amounts of energy. It is necessary that the beams be directed with extreme accuracy over distances of thousands of miles, and that they be carefully controlled to avoid losses from convergence, divergence or aberrations. It has been difficult to develop a system capable of sensing these spatial characteristics of a high energy neutral particle beam, partly because sensing apparatus that occlude any significant portion of the beam tend to be destroyed because of excessive heating from the beam.
Neutral particle beams can be formed from atoms of a variety of different elements, the particular type of particle selected depending upon the end use for the beam. The beams are typically generated by accelerating negatively charged particles in a nuclear particle accelerator to a very high energy level. The particles are charge-neutralized just prior to release from the accelerator by removing the excess electrons. This is accomplished either by passing the charged beam through a dilute curtain of gas which strips off electrons, or by using a resonant laser beam to excite extra electrons sufficiently to remove them. The length of the beam path through the electron stripping medium together with the gas density or laser strength determines the amount of electron stripping; the result of each method is often a mix of neutral and charged beam particles.
Two approaches are presently being investigated to sense the direction of a neutral particle beam. The first is an edge sensing technique that uses a "pin hole" sensor. This is essence is a lead plate with a series of pin holes that is placed around the edge of the beam and emits a series of pin hole beams. The pin hole beams are small enough in diameter so that their directions can be traced by placing a target in their path and detecting where they strike the target. The direction of the pin hole beams, and thus of the entire beam, can then be determined by comparing the location of the pin hole beams at the target with the locations of the source pin holes. The pin hole approach is not believed to have been demonstrated to date, but it is expected to have operational problems since only the edges of the beam are sensed, and the known aberrations in the beam shaping optics are likely to give the wrong beam directions from the edges.
The second approach being investigated uses a laser resonance fluorescence (LRF) sensor. The LRF concept aligns a laser to the particle beam at a large angle such as 54.degree., after which the laser beam needs to be rotated to the particle beam line of sight with accuracies in the order of 1:1,000,000. The 54.degree. angle is dependent upon the beam energy and laser wave length, and is difficult to maintain as the particle beam is pointed. An additional limitation of the LRF approach is that it is dependent upon a weak optical interaction, which is likely to result in serious limitations for high bandwidth control loops.
In addition to accurately determining the beam direction, it is highly desirable that a technique be developed for reliably determining the beam's divergence or convergence (the term "divergence" will be used hereinafter to include both positive divergence, and convergence as a negative divergence). As it is generated, a neutral particle beam is expanded and recollimated and can have an undesirably high divergence as it emerges from the expansion optics. It would be desirable to restrict any divergence to less than 10 microradians, but to do this an accurate mechanism for sensing divergence must be developed.